The present invention relates to medical knifes used in surgeries for cutting biological tissue such as eye tissue.
FIGS. 14 and 15 each show a conventional medical knife described in Japanese Laid-Open Patent Publication No. 2001-238890. The medical knife includes a pair of first cutting surfaces and a first intermediate surface 106c formed at a first side 106 of a blade 101. In each first cutting surface, an outer slanted surface 106a and an inner slanted surface 106b are defined. A hypothetical plane 105 extends between a pair of cutting edges 103 of the blade 101. The angle between the hypothetical plane 105 and each of the inner slanted surfaces 106b, that is a semi cutting angle of the inner slanted surface 106b, is smaller than the angle between the hypothetical plane 105 and each of the outer slanted surfaces 106a, that is a semi cutting angle of the outer slanted surface 106a. 
Also, a pair of opposed second cutting surfaces are formed at a second side 107 of the blade 101. In each second cutting surface, a slanted surface 107a is defined between the cutting edge 103 and the contour line 107c. The angle between each slanted surface 107a and the hypothetical plane 105, that is a semi cutting angle of the slanted surface 107a, is uniform. A planar surface 107b extends between the contour lines 107c and parallel with the hypothetical plane 105.
Thus, if the cutting edges 103 are formed relatively sharp, the thickness between the first side 106 and the second side 107 of the blade 101 becomes relatively small, seconding the strength of the blade 101. However, if the thickness between the first side 106 and the second side 107 is maintained at a relatively large value for saving the strength of the blade 101, the sharpness of each of the cutting edges 103 becomes insufficient. This limits the cutting performance of the medical knife.